See Una affected by tension (6)
I believe the answer is:
unease
'tension' is the definition.
(I've seen this in another clue)
'see una affected' is the wordplay.
'affected' indicates an anagram.
'see'+'una'='seeuna'
'seeuna' is an anagram of 'UNEASE'.
'by' acts as a link.
(Other definitions for unease that I've seen before include "Disquiet" , "State of anxiety" , "Discomfort or apprehension" , "State of worry and lack of confidence" , "Anxiety, discontent" .)